CWE-400: Uncontrolled Resource Consumption ('Resource Exhaustion')

The software does not properly restrict the size or amount of resources that are requested or influenced by an actor, which can be used to consume more resources than intended.

Extended Description

Limited resources include memory, file system storage, database connection pool entries, or CPU. If an attacker can trigger the allocation of these limited resources, but the number or size of the resources is not controlled, then the attacker could cause a denial of service that consumes all available resources. This would prevent valid users from accessing the software, and it could potentially have an impact on the surrounding environment. For example, a memory exhaustion attack against an application could slow down the application as well as its host operating system.

Resource exhaustion problems have at least two common causes:

Error conditions and other exceptional circumstances

Confusion over which part of the program is responsible for releasing the resource

The most common result of resource exhaustion is denial of service.
The software may slow down, crash due to unhandled errors, or lock out
legitimate users.

Access Control

Other

Technical Impact: Bypass protection
mechanism; Other

In some cases it may be possible to force the software to "fail open"
in the event of resource exhaustion. The state of the software -- and
possibly the security functionality - may then be compromised.

Likelihood of Exploit

Medium to High

Detection Methods

Automated Static Analysis

Automated static analysis typically has limited utility in recognizing
resource exhaustion problems, except for program-independent system
resources such as files, sockets, and processes. For system resources,
automated static analysis may be able to detect circumstances in which
resources are not released after they have expired. Automated analysis
of configuration files may be able to detect settings that do not
specify a maximum value.

Automated static analysis tools will not be appropriate for detecting
exhaustion of custom resources, such as an intended security policy in
which a bulletin board user is only allowed to make a limited number of
posts per day.

Effectiveness: Limited

Automated Dynamic Analysis

Certain automated dynamic analysis techniques may be effective in
spotting resource exhaustion problems, especially with resources such as
processes, memory, and connections. The technique may involve generating
a large number of requests to the software within a short time
frame.

Effectiveness: Moderate

Fuzzing

While fuzzing is typically geared toward finding low-level
implementation bugs, it can inadvertently find resource exhaustion
problems. This can occur when the fuzzer generates a large number of
test cases but does not restart the targeted software in between test
cases. If an individual test case produces a crash, but it does not do
so reliably, then an inability to handle resource exhaustion may be the
cause.

Effectiveness: Opportunistic

Demonstrative Examples

Example 1

(Bad Code)

Example
Language: Java

class Worker implements Executor {

...

public void execute(Runnable r) {

try {

...

}

catch (InterruptedException ie) {

// postpone response

Thread.currentThread().interrupt();

}

}

public Worker(Channel ch, int nworkers) {

...

}

protected void activate() {

Runnable loop = new Runnable() {

public void run() {

try {

for (;;) {

Runnable r = ...;

r.run();

}

}

catch (InterruptedException ie) {

...

}

}

};

new Thread(loop).start();

}

}

There are no limits to runnables. Potentially an attacker could cause
resource problems very quickly.

Example 2

This code allocates a socket and forks each time it receives a new
connection.

(Bad Code)

Example Languages: C and C++

sock=socket(AF_INET, SOCK_STREAM, 0);

while (1) {

newsock=accept(sock, ...);

printf("A connection has been accepted\n");

pid = fork();

}

The program does not track how many connections have been made, and it
does not limit the number of connections. Because forking is a
relatively expensive operation, an attacker would be able to cause the
system to run out of CPU, processes, or memory by making a large number
of connections. Alternatively, an attacker could consume all available
connections, preventing others from accessing the system
remotely.

Example 3

In the following example a server socket connection is used to
accept a request to store data on the local file system using a specified
filename. The method openSocketConnection establishes a server socket to
accept requests from a client. When a client establishes a connection to
this service the getNextMessage method is first used to retrieve from the
socket the name of the file to store the data, the openFileToWrite method
will validate the filename and open a file to write to on the local file
system. The getNextMessage is then used within a while loop to continuously
read data from the socket and output the data to the file until there is no
longer any data from the socket.

(Bad Code)

Example Languages: C and C++

int writeDataFromSocketToFile(char *host, int port)

{

char filename[FILENAME_SIZE];

char buffer[BUFFER_SIZE];

int socket = openSocketConnection(host, port);

if (socket < 0) {

printf("Unable to open socket connection");

return(FAIL);

}

if (getNextMessage(socket, filename, FILENAME_SIZE) >
0) {

if (openFileToWrite(filename) > 0) {

while (getNextMessage(socket, buffer, BUFFER_SIZE)
> 0){

if (!(writeToFile(buffer) > 0))

break;

}

}

closeFile();

}

closeSocket(socket);

}

This example creates a situation where data can be dumped to a file on
the local file system without any limits on the size of the file. This
could potentially exhaust file or disk resources and/or limit other
clients' ability to access the service.

Example 4

In the following example, the processMessage method receives a two
dimensional character array containing the message to be processed. The
two-dimensional character array contains the length of the message in the
first character array and the message body in the second character array.
The getMessageLength method retrieves the integer value of the length from
the first character array. After validating that the message length is
greater than zero, the body character array pointer points to the start of
the second character array of the two-dimensional character array and memory
is allocated for the new body character array.

(Bad Code)

Example Languages: C and C++

/* process message accepts a two-dimensional character array of
the form [length][body] containing the message to be processed
*/

int processMessage(char **message)

{

char *body;

int length = getMessageLength(message[0]);

if (length > 0) {

body = &message[1][0];

processMessageBody(body);

return(SUCCESS);

}

else {

printf("Unable to process message; invalid message
length");

return(FAIL);

}

}

This example creates a situation where the length of the body
character array can be very large and will consume excessive memory,
exhausting system resources. This can be avoided by restricting the
length of the second character array with a maximum length check

Also, consider changing the type from 'int' to 'unsigned int', so that you are always guaranteed that the number is positive. This might not be possible if the protocol specifically requires allowing negative values, or if you cannot control the return value from getMessageLength(), but it could simplify the check to ensure the input is positive, and eliminate other errors such as signed-to-unsigned conversion errors (CWE-195) that may occur elsewhere in the code.

(Good Code)

Example Languages: C and C++

unsigned int length = getMessageLength(message[0]);

if ((length > 0) && (length <
MAX_LENGTH)) {...}

Example 5

In the following example, a server object creates a server socket
and accepts client connections to the socket. For every client connection to
the socket a separate thread object is generated using the
ClientSocketThread class that handles request made by the client through the
socket.

(Bad Code)

Example
Language: Java

public void acceptConnections() {

try {

ServerSocket serverSocket = new
ServerSocket(SERVER_PORT);

int counter = 0;

boolean hasConnections = true;

while (hasConnections) {

Socket client = serverSocket.accept();

Thread t = new Thread(new
ClientSocketThread(client));

t.setName(client.getInetAddress().getHostName() + ":"
+ counter++);

t.start();

}

serverSocket.close();

} catch (IOException ex) {...}

}

In this example there is no limit to the number of client connections
and client threads that are created. Allowing an unlimited number of
client connections and threads could potentially overwhelm the system
and system resources.

The server should limit the number of client connections and the
client threads that are created. This can be easily done by creating a
thread pool object that limits the number of threads that are
generated.

Chain: anti-virus product encounters a malformed file but returns from a function without closing a file descriptor (CWE-775) leading to file descriptor consumption (CWE-400) and failed scans.

Potential Mitigations

Phase: Architecture and Design

Design throttling mechanisms into the system architecture. The best
protection is to limit the amount of resources that an unauthorized user
can cause to be expended. A strong authentication and access control
model will help prevent such attacks from occurring in the first place.
The login application should be protected against DoS attacks as much as
possible. Limiting the database access, perhaps by caching result sets,
can help minimize the resources expended. To further limit the potential
for a DoS attack, consider tracking the rate of requests received from
users and blocking requests that exceed a defined rate threshold.

Phase: Architecture and Design

Mitigation of resource exhaustion attacks requires that the target
system either:

recognizes the attack and denies that user further access for a
given amount of time, or

uniformly throttles all requests in order to make it more
difficult to consume resources more quickly than they can again be
freed.

The first of these solutions is an issue in itself though, since it
may allow attackers to prevent the use of the system by a particular
valid user. If the attacker impersonates the valid user, he may be able
to prevent the user from accessing the server in question.

The second solution is simply difficult to effectively institute --
and even when properly done, it does not provide a full solution. It
simply makes the attack require more resources on the part of the
attacker.

Phase: Architecture and Design

Ensure that protocols have specific limits of scale placed on
them.

Phase: Implementation

Ensure that all failures in resource allocation place the system into
a safe posture.

Other Notes

Database queries that take a long time to process are good DoS targets. An
attacker would have to write a few lines of Perl code to generate enough
traffic to exceed the site's ability to keep up. This would effectively
prevent authorized users from using the site at all. Resources can be
exploited simply by ensuring that the target machine must do much more work
and consume more resources in order to service a request than the attacker
must do to initiate a request.

A prime example of this can be found in old switches that were vulnerable
to "macof" attacks (so named for a tool developed by Dugsong). These attacks
flooded a switch with random IP and MAC address combinations, therefore
exhausting the switch's cache, which held the information of which port
corresponded to which MAC addresses. Once this cache was exhausted, the
switch would fail in an insecure way and would begin to act simply as a hub,
broadcasting all traffic on all ports and allowing for basic sniffing
attacks.